This invention relates to a method of forming or shaping a bearing bushing having an inner sliding surface used in various kinds of machines, such as an automobile, an industrial machine and an agricultural machine, and more particularly to a bushing used in that portion of such a machine which is liable to receive an impact load and an uneven load.
For example, a bushing, mounted as a bearing member in a shock absorber provided at a front fork of a motorcycle, receives an impact at the time of start of the motorcycle, and also is greatly influenced by undulations on the road during running of the motorcycle. Thus, due to vibrations transmitted via wheels at the time of the start and during running, as well as bending of an inner tube (or a shaft) slidably engaged within the bushing, it is desirable that the sliding surface of the bushing should be formed into a crown or crowning shape so that the impact load and the even load can be absorbed or alleviated.
There are known methods of providing a crown or crowning shape to an inner peripheral surface of a bushing so that the inner diameter of the bushing is larger at its opposite longitudinal or axial ends, and decreases progressively toward the central zone of the bushing. In one such known method, a circular internal hole of a housing in which a bushing is to be mounted is machined into a smaller diameter at its axial central zone, and also is machined into a larger diameter at its opposite end zones, and then the bushing is inserted into the housing, and subsequently the bushing is expanded and deformed into conformity with the peripheral surface of the circular internal bore hole of the housing. In another such known method using expensive machining, only those zones of an inner peripheral surface of a bushing adjacent to opposite ends of the bushing are machined to provide a crowning shape to the inner peripheral surface.